HANOI, Vietnam — The Vietnamese government has lashed out against the presence of a Chinese oil rig in the disputed waters of the South China Sea, the latest in what Vietnam says are a series of provocative actions by Beijing this month.

While the dispute raised tensions between the Communist neighbors, there were no signs yet of the heated escalation that characterized a similar episode in 2014, when relations between the two countries plummeted and anti-Chinese demonstrations spiraled into deadly riots.

Late on Tuesday, the Vietnamese Foreign Ministry said that Haiyang Shiyou 981, the same rig that was at the center of the 2014 feud, had entered disputed waters in the South China Sea on Saturday, according to a statement on the ministry’s website.

The rig was still 25 miles from an “assumed median line” between the two countries, the statement said, but it was in “an overlapping area between the two continental shelves” of Vietnam and Hainan Island, China, which “has not yet been delimited.”

A Vietnamese official met with a Chinese Embassy official on Monday to register Vietnam’s “concern,” the statement said. It added that China should remove the rig from the disputed waters in accordance with international law.

China insisted that the rig was still in its territorial waters.

“To our knowledge, China’s Haiyang Shiyou 981 drilling platform is working in totally indisputable waters under China’s jurisdiction,” said Hong Lei, a Chinese Foreign Ministry spokesman, at a news conference on Wednesday. “It is hoped that the Vietnamese side can view it calmly, work with China in the same direction and make joint efforts to properly handle the maritime issue.”

The dispute came as Vietnam’s top leaders convened here on Wednesday for the start of a Communist Party national congress, which will choose the country’s leaders for the next five years. Analysts said the dispute was unlikely to affect those decisions, and the party appeared to be taking pains not to alienate China.

But the presence of the oil rig has raised anxiety here, and it comes after several other diplomatic scrapes.

Vietnam asked China to investigate the ramming of a Vietnamese fishing vessel this month by a boat that the captain said was marked with Chinese characters. In recent weeks, Vietnam has also complained about several unannounced, state-sponsored Chinese flights through Vietnamese-administered airspace in the South China Sea.

Also this month, Vietnam formally accused China of violating its sovereignty, as well as a recent confidence-building pact, after Beijing landed a plane on an artificial island built by China.

“Speculation as to whether and how the timing of these actions might affect Vietnam’s leadership succession misses the more glaring point that Beijing appears not to care about international norms or Vietnamese claims and sensibilities,” Jonathan London, a Vietnam expert in the Department of Asian and International Studies at the City University of Hong Kong, said in an email.

Le Hong Hiep, a visiting fellow at the ISEAS-Yusof Ishak Institute in Singapore, said the timing of the oil rig’s movements — at the moment when Vietnam begins a twice-a-decade power transfer — may be a coincidence. But whatever the reason, he added, Vietnam is unlikely to immediately “take strong actions that will cause tension,” such as sending Coast Guard ships to the area to challenge the oil rig, as it did in 2014.

“The party wants to make sure the party congress is a success,” he said.

The oil rig, China’s first domestically built mobile-drilling platform, is 449 feet tall and the covers an area the size of a football field. It is owned by the China National Offshore Oil Corporation, a state-owned oil giant that handles most of China’s offshore drilling, according to a report last month in Global Times, a state-run Chinese newspaper.

Starting in May 2014, the rig lingered for weeks in disputed waters close to the disputed Paracel Islands and the central Vietnamese coast.

Vietnam and China have been attempting to mend their relations ever since, but the episode generated a heated national debate among Vietnamese about the country’s political and economic dependence on its giant northern neighbor. Analysts say it also accelerated Vietnam’s long-running effort to improve relations with the United States and other global powers.

A pending reshuffle of party leadership, to be decided at the congress over the next week, is said to hinge on an internal power struggle between Nguyen Phu Trong, the party’s incumbent general secretary, and Nguyen Tan Dung, the prime minister. Mr. Dung is generally seen as more friendly to American officials and the international business community, although Mr. Trong visited the White House last year and has supported Vietnam’s membership in the American-led Trans-Pacific Partnership deal.

Mr. Hiep said the rig appeared to be on the Chinese side of a line running between Hainan, a Chinese resort island, and the Vietnamese central coast, but he added that Vietnam may take stronger actions if the rig crossed the line, although not until after the party congress ends in late January.

Nguyen Hung Cuong, a lawyer and South China Sea expert at Vietnam’s Scientific Research Institute of Sea and Islands in Hanoi, said the rig’s return to disputed waters appeared to be a deliberate escalation by China to claim vast areas of territory in the South China Sea.

But Mr. Cuong said it was unclear as of Wednesday whether the oil rig was just passing through to exercise freedom of navigation, or whether it had already started to drill.